Various types of control of a synchronous motor based on a primary magnetic flux, which is the so-called primary magnetic flux control, have been proposed. Briefly stated, the primary magnetic flux control is control of the primary magnetic flux of the synchronous motor in accordance with a command value thereof (hereinafter, referred to as a “primary magnetic flux command”) to control a phase difference (load angle) of a phase of the primary magnetic flux from a phase of a field magnetic flux so that the phase difference becomes a predetermined phase difference, for example. Specifically, a command value of a rotational speed (hereinafter, “rotational speed command”) of the synchronous motor and the primary magnetic flux command are controlled to control a voltage applied to the synchronous motor, to thereby indirectly control a current flowing through the synchronous motor and, further, torque to obtain a desired rotational speed.
Japanese Patent No. 5494760 proposes technology for correcting a deviation of the load angle from the predetermined phase difference. Patent Japanese Patent No. 5556875 proposes technology for generating the primary magnetic flux command. Japanese Patent No. 2551132 proposes technology for controlling the current flowing through the synchronous motor so that the current becomes constant. Japanese Patent No. 3874865 and WO 2003/071672 propose technology for controlling the torque of the synchronous motor.